Less than two months after edging out Best Buy and Target for the number two spot, Apple (NSDQ: AAPL) says it has now passed Wal-Mart (NYSE: WMT) as the number one largest music retailer in the U.S. Citing NPD Group’s MusicWatch survey, Apple claims five-year-old iTunes has sold over four billion songs to more than 50 million customers. Apple also claims its catalog of six million-plus songs is the world’s largest. NPD bases its findings on the amount of music sold during January and February 2008. Release.

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It could be coincidental that the news happened to hit the same day that MySpace unveiled its own new DRM-free music venture. But it probably had to do with Ars Technica, which got hold of some internal Apple emails saying how Apple actually reached the number one target in January.

According to the NPD data contained in the emails, the sales volume breaks down as this: iTunes has 19 percent, followed by Wal-Mart’s combined brick-and-mortar outlets and online sales with 15 percent; Best Buy is third with 13 percent and Amazon (NSDQ: AMZN) lags fourth at 6 percent. The list continues with Borders, Circuit City, and Barnes & Noble; Rhapsody is number 10 with 1 percent. NPD shows that Apple still has far to go, as the “other” slot has 28 percent. Ars points out that Apple’s ascent to the number one position was helped by gift cards being activated after the Christmas holidays, though Apple isn’t the only retailer that offers such cards.

The news outlines more trouble for the record industry, as iTunes customers tend to pick and choose individual tracks rather than buy whole albums, meaning less money for the labels.